Process for electron sterilization of a container

ABSTRACT

Minimum electron energy is used for the sterilization of preformed containers in order to minimize the machinery size, cost and radiation shielding required for in-line use. The electron energy required is reduced by the use of one-sided (unilateral) irradiation wherein the dose delivered by the primary radiation to thicker portions of the containers is supplemented by the dose delivered by scattered electrons.

This application claims the benefit of provisional application Ser. No.60/401,122, filed Aug. 5, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to electron processing, and in particular to thesterilization of containers by energetic electrons.

2. Description of the Related Art

The application of energetic electrons to the sterilization offood/pharmaceutical containers has attracted considerable effort in thepast⁽¹⁾, much of which has focused on treatment of the food contactsurfaces in form-fill-seal⁽²⁾ or the treatment of preformed containerstumble packed and bagged⁽³⁾. Techniques have been published^((4, 5)) onthe in-line electron treatment of containers, typically blown polymers,for application to high speed filling lines. One of the major problemsfacing the adaptation of such a process to pre-sterilized (aseptic oresl, extended shelf life) filling machinery is the requirement ofproviding a completely sterile container to the filling equipment. Twomajor problems exist in this application:

-   -   1. The need to support the container in order to control its        transport through the sterilizer and    -   2. The need to control its position and vertical orientation        into the filling machine, usually of rotary design.

If inadequate electron energies are used for full penetration of thecontainer walls as in the low energy processes taught earlier⁽²⁾, thenspecial precautions are necessary in the filler in order to prevent anycontamination of the contents by microorganism convective transport tothe fill spout/container mouth area or transfer contamination of thepre-sterilized filler region itself by surface borne microorganisms.

Pneumatic transport techniques have been developed⁽⁶⁾ and are widelyused on such equipment by which the container is moved alonghorizontally on rails located in grooves blown or molded into thecontainer for this purpose, usually under the container's neck. The factthat these rails must remain in contact with bottles moving at up to 1m.sec⁻¹ in a high speed filler, means that no time is available forbottle rotation or exposure of the rail contact areas and, of course,these sections exposed to the electron beam must be water cooled fordissipation of the electron energy absorbed in the rails.

If a more traditional conveyor is employed, using vacuum hold-down ofthe erect bottles, for example, then the interface between the bottlebottom and the conveyor plate is inaccessible to the sterilizing flux ofenergetic electrons. Hence its transport from the in-line sterilizer tothe filler can lead to fill-zone contamination. Techniques are availablefor HEPA or sterile air isolation of the fill-spout/container neck area,but the possibility still exists for the transport of viablemicroorganisms remaining on the container bottom to the critical,pre-sterilized regions of the filler.

This application teaches techniques, verified experimentally withelectron dosimetry, which permit complete sterilization of blow-moldedbottles and other open-mouthed containers. It employs containerpresentation techniques which allow electron access to all surfaces ofthe container and which do not require rigid gripping or the use ofdevices which block electron access to those contacted surface areas ofthe container during the sterilization process.

Current practice for bottle sterilization/disinfection utilizes liquiddisinfectants such as paracetic acid and/or hydrogen peroxide, or theymay be applied in a spray form. For liquid treatment this requires anextended holding time (8-10 s) of the solution in the container in orderto be efficacious, followed by washing and drying to remove residualcontaminants on the food contact surfaces. This sequence required largeaccumulation areas at the filling speeds of interest (e.g. 10 sec⁻¹ at16 oz) and special environmental considerations for handling of thewastewater. Present systems utilize large star wheels, typically withneck grippers, for transport of the containers in a vertical orientationthrough the above steps and into the pre-sterilized filler region of thesystem.

SUMMARY OF THE INVENTION

This invention comprehends a process for electron sterilization of allsurfaces of open-mouthed containers, cups or bottles, where thesterilizing energy is directed laterally at the sidewall of thecontainer in a sheet extending beyond the ends of the container, so thatthe dose delivered by the primary radiation to both the exterior andinterior surfaces of the container is supplemented by scatteredradiation.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention may best be understood from the following detaileddescription thereof, having reference to the accompanying drawings, inwhich:

FIG. 1 is a graph showing typical lead thickness as a function ofoperating voltage for a 20 mA machine;

FIG. 2 is a graph showing depth dose for 175 kV-700 kV electron beamprocessors;

FIG. 3 is a diagram showing unilateral sterilization geometry for fluidcontainers;

FIG. 4 is a diagram showing top outer and inner surface dosimetry at 475kV;

FIG. 5 is a diagram showing bottom outer and inner surface dosimetry at475 kV;

FIG. 6 is a diagram showing upper handle mouth region of 3-quart jugtreated as in FIG. 3;

FIG. 7 (FIGS. 7 a and 7 b) is a diagram showing bottom exterior andinterior regions of 3-quart jug treated as in FIG. 3; FIG. 7 b is adiagrammatic rearrangement of the dose data of FIG. 7 a;

FIG. 8 is a diagram showing a vacuum bottle/jug gripper with ends free;

FIG. 9 is a diagram showing a spring-loaded gripper with ends free;

FIG. 10 is a diagram showing a general starwheel arrangement forelectron sterilization;

FIG. 11 is a diagram showing a neck gripper opened in sterilization zoneonly;

FIG. 12 is a diagram showing self-shielded bottle transport; and

FIG. 13 is a diagram showing shielded tunnel geometry with mechanicalgrippers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Purpose of the Process

The teaching of this application is directed to the use of minimumelectron energy for the sterilization of preformed containers in orderto minimize the machinery size, cost and radiation shielding requiredfor in-line use. These sterilizers are designed to permit use inuncontrolled areas which means that the X-ray levels at the surfaces forthe system must be reduced to levels acceptable for continuous humanoccupancy, typically 0.05 millirem/h or 0.5μ Gy/h⁽⁷⁾. An indication ofthe lead thicknesses required to reduce a sterilizer of modest power tothese levels is shown⁽⁸⁾ in FIG. 1. The goal of the system designer thenis to arrive at an operating voltage for the accelerator which provideselectrons of adequate energy to sterilize all surfaces of the containerwhile subjecting those regions which are easily treated, such as thecontainer's outer surface, to dose levels which are acceptable. That is,a dose uniformity is sought which provides an acceptable maximum dosewhen the minimum treatment level is that required by the filling/cappingprocess. Other work⁽⁹⁾ with B. pumilus (ATCC 27142) has shown this to be10 kGy for the 6 Log Count Reduction (LCR) required for aseptic filling.Such a dose requires the deposition of 2.4 calories/g to the containersurfaces. High non-uniformity, say ten or twenty to one, might thereforelead to energy deposition levels capable of thermal deformation ofpolymer containers, or excessive color center formation in glass. Suchexcess levels are encountered in processes such as described in U.S.Pat. No. 3,780,308 or in U.S. Pat. No. 6,221,216 where large volumecontainers must be treated through small area openings (that is, thefill mouth). A novel approach involving isotropic irradiation of acontainer for low speed (6 min⁻¹) container sterilization has beendescribed by Sadat and Huber⁽¹⁰⁾ but such an approach is impractical forhigh speed (100-1000 min⁻¹) filling applications addressed here. Morecomplex bilateral irradiation techniques have been described for thesterilization of tubular containers⁽⁴⁾ but have not been commercialized.

Requirement of the Process

The need to achieve a minimum surface treatment⁽⁹⁾ with one-sided(unilateral) irradiation is challenged by the thicknesses for thecontainer walls (in particular the thick bottom of blow molded polymerbottles or jugs) and the heavy walls of structurally strengthenedregions of the container—such as the neck and, if screw capped, threadedregions. Because the main interest in the application of this art is inthe filling of 250 ml to 4 liter containers for liquid foodstuffs, atypical 1 liter polyester bottle will be used to illustrate theunexpected features of the process. The wall thicknesses over the 3-cmlong neck region of such a container will range from 0.16 to 0.23 cm inthickness or, for polyester, for example, with a density of 1.2 g.cm⁻³,a weight per unit area of 1900-2800 g.m⁻². Areas in the bottom of blowmolded polymer containers can range above these figures, to 3600 g.m⁻²or more.

The penetration of electrons in matter varies with this measure of arealdensity since the deposition of energy by the primary electron isdetermined by multiple electron-electron collisions. The electrondensity in matter is measured by this areal density figure, so thatpenetration depths are given (and displayed) in these units. FIG. 2shows some measured⁽¹²⁾ penetration curves for electron sterilizers(accelerators) working at various energies. One can see that very highvoltages (energies) are involved in the penetration of matter above 1000g.m⁻², for example 6-700 kV. Such voltages require heavy shieldconfigurations as shown in FIG. 1. It is therefore obvious thatunilateral electron treatment at energies below full penetrationcapability might be expected to offer very non-uniform exterior-interiorside wall and bottom treatment as well as inadequate interior necktreatment.

Preferred Process Geometry

Using 9 μm film dosimetry,⁽¹³⁾ dose surveys over all surfaces of acontainer have been made using unilateral irradiation of the containeralong its longitudinal axis. This would typically utilize a scannedelectron beam although a “curtain type” beam would be equallyapplicable⁽⁴⁾. This arrangement would typically involve the transport ofthe container through a beam whose transverse width is smaller than thatof the container but whose longitudinal length is considerably longerthan the container's height so that air scattered electrons bathe theexterior bottom and mouth opening in a relatively uniform manner. Thegeometry is shown in FIG. 3.

The experimental dose distributions along the front (top) surface of theneck and its top interior are shown in FIG. 4 at a 475 kV operatingvoltage. As shown in FIG. 2, this operating point offers a half dosepoint of −900 g.m⁻² and an end or maximum penetration of only 1400g.m⁻², well below that required for neck or bottom penetration. However,the surface dose distributions for both regions under these operatingconditions as shown in FIG. 4 are relatively uniform due to thescattering geometry offered by the container irradiation geometryillustrated in FIG. 3. For the 1-liter bottle shown, the uniformity was±10% over the top exterior 6 cm from the mouth plane, and ±30% over thetop interior 8 cm from the mouth plane.

Under these same conditions, the interior and exterior lower surfacedose profiles are shown in FIG. 5. Here the interior lower surfacedemonstrated a uniformity of ±7% and an exterior rear (lower) surfaceuniformity of ±30% over 7 cm from the mouth plane. These resultsrevealed the unexpected result that electrons incapable of fullpenetration of the thicker regions of the container could provide quiteuniform treatment of the “protected” neck regions using the longitudinaltreatment geometry.

Experimental studies conducted at 475 kV in a scanned machine with thepreferred longitudinal orientation shown in FIG. 3, used 6 mm diametersupport rails in order to simulate the “air conveyor” or pneumatictransport geometry. With the use of film dosimetry to map the railcontact interface on the outer surface of the container about its 360°periphery, it was found that with the 32 ounce container, themaximum/minimum dose ratio was reduced to under 2:1 with containerrotation versus the 5:1 ratio measured with static, unilateraltreatment.

Similar results using the identical dosimetric film mapping techniqueare shown in FIGS. 6 and 7. FIG. 6 shows the dimensions of a 3 quartpolyester jug with a hollow (normally liquid filled) handle whose wallsare 1300 g.m⁻² thick and whose side wall range from 600-1200 g.m⁻². Theelectron beam longitudinal direction is shown by vectors AB, while thedirection of transport of the container through the beam is shown byvector C of FIG. 6. The dose data shown for single pass treatment reveala ±30% uniformity around the interior handle, and the same behavior ofthe dose distribution around the mouth opening as in the case of the 32ounce bottle. The interior to exterior rear wall dose ratio was found tobe 3:1, a figure much better than expected for the 1300-1400 g.m⁻² ofmaterial presented to the beam. The “side presentation” of the containerto the beam was found to be necessary to improve the treatmentuniformity at the handle plane.

FIG. 7 illustrates the bottom interior and exterior dose distributions.The exterior distribution shows excellent uniformity in both radialdirections, while the interior shows similar uniformity (±10%). What isagain unexpected is the good exterior to interior dose ratios ofapproximately unity. For the conditions employed in these trials at 550kV operating voltage, the average treatment level throughout allsurfaces of the jug was 25 kiloGrays. The lowest dose measured of 5 kGywas on the rear surface in the upper portion of the container. If oneselects a minimum dose of 10 kGy for aseptic application, the overalldose would be doubled and an average surface dose (interior andexterior) of 50 kGy would result. Related extractables studies haveshown this (upper) level to be quite acceptable for the widely used blowmolded polymers, such as high density polyethylene and polyester, forbottle/jug manufacture.

For practicing the invention in commercial applications one would use avacuum gripper which holds the bottle by its thin sidewall, so that theforward directed electrons can penetrate directly to the rear surfaceand deliver a sterilizing dose to that rear exterior surface immediatelyadjacent to the gripper.

This arrangement is shown in FIG. 8 and holds the container in thetreatment or sterilization zone so that both mouth opening and bottomsurface are free and unblocked by any support or retainer mechanisms.

An alternate mechanical gripper is shown in FIG. 9, in which a190-degree-200-degree spring-loaded gripper holds the bottle, again onits thinner central section where complete penetration of two wallthicknesses at the chosen sterilizer energy is feasible.

FIG. 10 shows the general arrangement for feeding the bottles using aconventional starwheel A to feed containers to the central wheel B.Pre-sterilization of the system is only required for wheels B and C (thetake-off wheel) and the conveyor into the filler, usually a rotary typesystem which could be located on wheel C. Conventional gripping of thecontainer (typically in the thicker walled neck region) is employed in Aand C, while back-surface gripping described herein and shown in FIGS. 8and 9 is used in wheel B to conduct the containers from A and to C (i.e.through the sterilization zone). The gripper sequence or spacing can bedesigned to accommodate containers of varying widths, so that, forexample, wheels A through C can handle containers (e.g. 3 liter) oftwice the width of the 1 liter container at half the throughput rate,with appropriate adjustment of the filler spout sequencing. Once again,after sterilization the container may immediately return to aconventional neck gripper for transfer off to starwheel C, so that, asshown in FIG. 11, the neck gripper need only be opened in the electronsterilization zone itself Hence, starwheel B offers both grippingfixtures, only resorting to the rear-surface gripping in the ebtreatment zone so that the containers are dependent on vacuum grippingalone over a small portion of their circular transport on B: i.e. lessthan or equal to 10 degrees. This mechanism allows the use ofwell-proved neck gripping techniques to be utilized in the electronsterilization process of the invention.

The invention includes unilateral electron beam treatment of thecontainer as described hereinabove with a vertical container motion sothat no contact is required with the container while in the treatmentzone, thus providing electron access to all surfaces. Such verticalcontainer motion is depicted in FIG. 12, in which the bottle dropsballistically or on a hold-down belt with no surface contact in awater-cooled duct into which the electron gun fires with an elongatedand vertically oriented beam. Such a configuration may also be used witha horizontally oriented beam (as shown) of higher dose rate throughwhich the container passes “ballistically” in its vertical drop frominfeed A to outfeed C. The major difficulty with this attractive(shielding) geometry is the re-gripping of the container aftersterilization, so that vacuum hold-down of the bottom of the verticallyoriented, now-sterile container can be used for conveyor transport aftersterilization. This transport system can be used for relocation by neckgripping on a filling/capping starwheel if desired, because thecontainer is now in a fully sterilized state.

A more conventional but less compact shielded tunnel geometry withmechanical hold-down on the chain (FIG. 9) is shown in FIG. 13.

The process of the invention is most usefully applied in thesterilization of molded containers possessing a thick walled base, athin walled central section, and a heavy walled neck and fill opening.This application includes both cylindrical (bottle) and rectangular(jug) geometries.

It is known that scattered electrons play an important role in energydeposition by either energetic electrons or gamma-rays. What is notobvious is the ability to sterilize all surfaces of a container in whichlarge portions of its surface consist of wall thicknesses far beyond therange of the primary electrons. In essence, applicants have demonstratedthe ability to choose a much reduced energy to effectively treat allsurfaces due to the scattering processes described and dependent uponthe ability of the primary beam to penetrate the intermediate sidewallsof the construction.

The teaching of the invention is made particularly valuable in the caseof the requirement of a fully sterile container for the pre-sterilizedfiller-capper region, such as for large volume aseptic packaging ofliquids. However, the invention also includes the vertical placement ofthe container with its exterior bottom “blocked”. This will beacceptable in certain applications where sterile air flow about the fillopening is used to “isolate” the interior of the container frommicroorganism contamination on the lower exterior surface . . . forexample in pharmaceutical filling.

In accordance with the invention, the multiply scattered secondaryelectrons are very important in the treatment of surfaces parallel tothe direction of motion of the primary electron beam. In the presentcase, they play the same important role in illumination of both bottomsurfaces, as well as uniform treatment of the interior and exteriorsurfaces of the thick walled neck and lip of the container.

The real issue with respect to the thickened portions is not blockingany portion of the exterior surface with grippers, support conveyors,etc., more because of the need for its unblocked exposure to direct andscattered electrons. As already discussed, the interior surfaces arebathed in air and container wall (polymer) scattered electrons, theprime sources of which are the primary electrons capable of penetratingthe thin central sidewalls of the containers.

The invention includes the following features:

-   1. Unilateral electron beam treatment of all surfaces of a container    with electron energies inadequate for full penetration of    diametrically opposed walls. The energy range utilized for the    process is for sterilizers operating in the 200-550 kV range.-   2. Unilateral electron beam treatment of all surfaces with the    longitudinal axis of the beam distribution oriented along the    container's longitudinal axis of symmetry.-   3. Unilateral electron beam treatment of the container oriented in    such a manner so as to prevent interior shielding of the food    contact surfaces by the molded handle or by sections of the    container walls which may be totally absorbing to the primary beam.-   4. Unilateral electron beam treatment of the container in the manner    described in 1 and 2, providing uniform treatment of the interior    bottom from electrons penetrating the side wall and of the exterior    bottom by air scatter from the primary beam.-   5. Unilateral electron beam treatment of the container in the manner    described in 1 and 2 with a vertical container motion so that no    contact is required with the container while in the treatment zone,    thus providing electron access to all surfaces.-   6. Unilateral electron beam treatment of the container in the manner    described in 1 and 2 with rotational motion of a vertical container    about its longitudinal symmetry axis so that regions of rail support    affecting electron access to the container surface are sufficiently    treated.-   7. Unilateral electron beam treatment of the container in the manner    described in 1 and 2 with fixed mechanical gripping or vacuum    hold-down of the container with good definition and control of the    sterile zone around the fill opening, so that any microorganisms    from these non-sterile exterior areas cannot reach the interior    surfaces during the filling and capping process.-   8. Physical organization of container handling machinery so that the    electron beam treatment zone defines the region before which    pre-sterilization is not necessary and beyond which    pre-sterilization and sterile operational maintenance are required.

REFERENCES

-   1. Nablo, S. V., Cleghorn, D. A. and Fletcher, P. M., “Dose    Distributions for Containers Electron Sterilized at Energies from    150-250 keV.”, Rad. Phys. Chem. 42, # 4-6, 827-831 (1993).-   2. Nablo, S. V., “Process and Apparatus for Surface Sterilization of    Materials”, U.S. Pat. No. 3,780,308, Dec. 18, 1973; Nablo, S. V. and    Cleghorn, D. A., “Technique for Interior Electron Sterilization of    an Open Mouthed Container”, U.S. Pat. No. 6,221,216, Apr. 24, 2001.-   3. Auslender, V. L. et al, “Automated Technological Radiation    Installation for Sterilization of Medical Goods”, Rad. Phys. Chem.    52, 459-465, (1998).-   4. Inai, T., Akai, T., Iwano, F., Yamamato, E. and Ueda, M.,    “Sterilization Treatment for Tubular Packaging Material”, Japanese    Patent Hei 11-35015, November (1999).-   5. Mittendorfer, J., Bierbaumer, H. P., Gratxl, F. and Kellauer, E.,    “Decontamination of food packaging using electron beam—status and    prospects,” Radiation Physics and Chemistry 63, 833 (2002).-   6. Private communication, Sentry Equipment Inc., 13150 East    Lynchburg Salem Tpke. Forest, Va. 24551.-   7. 29 CFR 1910. 1096, “Ionizing Radiation”, USGPO Washington (1996).-   8. “Safety Standard for Non-Medical X-Ray and Sealed Gamma-Ray    Sources”, Handbook 93, US Dept. of Commerce, Washington, D.C.    (1963).-   9. Cleghorn, D. A., Dunn, J. and Nablo, S. V., “Sterilization of    Plastic Containers Using Electron Beam Irradiation Directed Through    the Opening”, Journal of Applied Microbiology 93, 937-943 (2002).    See also Proc. WorldPak 2002, Vol 1, 81-90, MSU School of Packaging,    June 23-28, CRC Press New York (2002).-   10. Sadat, T. and Huber, T., “E Beam—a new transfer system for    isolator technology,” Radiation Physics and Chemistry 63, 587    (2002).-   11. “Charged Particle Range” section 3.6.3, Radiation Shielding    ed. J. K. Shultis and R. E. Faw, Prentice Hall PTR, N.J. (1996).-   12. “Practice for Dosimetry in an Electron Beam Facility for    Radiation Processing at Energies between 80 and 300 keV.”, ASTM    Standard E 1818-96, 2000 Annual Book of ASTM Standards, ASTM, 100    Barr Harbor Drive, PO Box C700, West Conshohocken, Pa. 19428-2959.-   13. FWT Radiachromic Film Type 60-810, Far West Technology Inc.,    330-D South Kellogg Avenue, Goleta, Calif. 93117.-   14. Unscanned electron processors of this type are manufactured and    sold under the trade names Electrocurtain® and EZ Cure® by Energy    Sciences Inc., 42 Industrial Way, Wilmington, Mass. 01867.

1. That method of irradiating, in a gaseous medium, a hollowopen-mouthed container having an axis and having walls of varyingthickness, which method comprises directing primary electrons laterallyagainst said container as a sheet extending along said axis and morethan the length of said axis, said primary electrons having energyinsufficient to penetrate through the entire container (i.e. two walls)at the thicker portions of said walls but sufficient to penetrate theentire container (i.e. two walls) at the thinner portions of said walls,the dose delivered to said thicker portions by said primary electronsbeing supplemented by secondary electrons produced by scattering of saidprimary electrons by said gaseous medium and said container.
 2. Methodof using reduced electron energy for the sterilization of preformedopen-mouthed containers, comprising the following steps: producing anelectron sheet in an electron-scattering gaseous region with across-sectional length L and width W, conveying an open-mouthedcontainer having an axis with a length less than L transversely throughsaid sheet in such a manner that the extremities of said sheet by-passsaid container while the region between said extremities passes throughsaid container, said axis being parallel to said sheet as said containerpasses through said sheet, the energy of said electrons being sufficientto provide the required minimum dose to all surfaces of said containerwhile remaining below that required for full penetration of the thickerregions of the container.
 3. Method in accordance with claim 2, whereinthe maximum treatment level delivered by said electrons remains belowthe threshold dose at which said electrons cause damage to saidcontainer.